The growing worldwide need of energy has expanded the search for oil and gas on the ocean floor to greater depths. At the present time it is contemplated to put oil and gas wells at ocean depths that only a few years ago were considered inaccessible.
At the same time there has been a tremendous concern about accidental leakage of gas and oil in the ocean and its effect on the sea life in particular and the environment in general. To take care of the need for gas and oil without adversely affecting the environment, it has been proposed that subsea wellhead cellars be utilized.
One such wellhead cellar system is disclosed in U.S. Pat. No. 3,601,189. This patent discloses a system in which a center pipe guide, tension member and a drilling template are used in a unique way to install the wells and to complete the wells. Specifically, the following steps are followed in installing the wellhead cellar of the preferred embodiment. After a bore is drilled in the desired area, a caisson is set in the bore and cemented in place. A drill template and guideline system is made as an integral part of the caisson. The template locates the upper ends of the wells in a closely spaced cluster, it being understood that the wells can be slant drilled, if desired. After all wells are drilled and capped an atmospheric wellhead cellar is lowered onto and sealed to the template. All water is then evacuated from the interior of the wellhead cellar by equipment carried by a personnel transporter. Thus, the wellhead cellar provides a single air chamber over the individual wells. A watertight personnel transporter operating with an internal pressure of one atmosphere can engage a hatch trunk on the wellhead cellar to permit personnel to be transported from the water surface to the interior of the wellhead cellar to permit the completion of the wells, installation of Christmas trees, flow lines and other necessary equipment to put the wells on-line.
The system shown in U.S. Pat. No. 3,601,189 performs quite well, except that it requires that all wells be drilled prior to the wellhead cellar being installed. Also, the number of wells that can be drilled and put on-line is restricted by the relatively small size of the wellhead cellar.
Moreover, it has been found desirable to isolate each well within its own chamber so that the well can be completed and reworked without disturbing the other wells.